In recent years, high-density data recording is promoted and large storage capacity is achieved in the field of disk drives, a representative example of which is the hard disk drive, owing to the development of magnetic heads of giant magneto-resistive effect (GMR) type and the development of perpendicular magnetic recording systems. Along with this technical development, the microwave assisted recording system which applies a high-frequency magnetic field to magnetic disks, has been proposed as a technique of increasing the recording density even more.
The microwave assisted recording system locally applies to a magnetic disk a magnetic field of high frequency much higher than the record signal frequency and near the resonance frequency of the magnetic disk. As a result, the magnetic disk undergoes resonance, decreasing the coercive force (Hc) at the surface of the magnetic disk to half the initial value or a smaller value. Therefore, data can be magnetically recorded on a magnetic disk having a larger coercive force (Hc) and high magnetic anisotropic energy (Ku) if a high-frequency magnetic field is superimposed on the recording magnetic field (see, for example, U.S. Pat. No. 6,011,664). However, the method disclosed in this document can hardly apply a high-frequency magnetic field at high efficiency in order to achieve high-density recording, because the high-frequency magnetic field is generated by a coil in this method.
In view of this, a method has been proposed, which utilizes a spin-torque oscillator (hereinafter referred to as “STO” as needed) as the source of the high-frequency magnetic field (see, for example, U.S. Pat. Appln. Publication No. 2005/0023938 or U.S. Pat. Appln. Publication No. 2005/0219771). The STO has a spin injection layer, an intermediate layer, an oscillation layer, and an electrode. In the technique disclosed in this document, a direct current is supplied through the electrode to the STO, whereby the spin injection layer generates a spin torque. The spin torque magnetizes the oscillation layer, which undergoes ferromagnetic resonance. As a result, the STO generates a high-frequency magnetic field.
The high-frequency magnetic field thus generated locally exists near the STO. Further, the magnetic disk perpendicularly magnetized can effectively undergo resonance by virtue of the in-plane component of the high-frequency field. The coercive force of the magnetic disk can therefore be greatly reduced. As a result, high-density, magnetic recording is performed, only at a part where the recording magnetic field generated by the main magnetic pole of the write head of the magnetic head is superimposed on the high-frequency magnetic field generate by the STO. A magnetic disk having both large coercive force (Hc) and high magnetic anisotropic energy (Ku) may be therefore utilized, thereby to avoid the problem of thermal fluctuation.
The drive current hitherto supplied to the STO is a direct-current signal of ordinary level. The STO has a delay time (oscillation delay time) that elapses until the STO starts oscillation after the write current corresponding to the data to be written on the magnetic disk has been inverted. The data transfer rate of the disk drive therefore increases. Hence, the STO cannot appropriately oscillate in some cases if the write current undergoes magnetization inversion at intervals shorter than the oscillation delay time of the STO. Consequently, the possibility of recording errors on the magnetic disk increases.
In such a case, a drive method in which a pulse signal component is superimposed on the direct ordinary-level drive current works well to shorten the polarity inversion time of the STO. If the drive current is increased very time the write data is inverted, however, the time the STO 30 is kept driven until it is broken will be short, inevitably resulting in a problem with the reliability of the STO.